1. Field of the Invention
The present invention relates to photon detectors and, more particularly, to a system for and method of correcting the pulse count output of each pixel of the photon detector as a function of the incident flux density.
2. Description of Related Art
Current state-of-the-art x-ray imaging systems employ scintillator photodiode arrays to detect and quantify x-rays and gamma rays after they pass through and are attenuated by an object under inspection. Scintillator photodiode arrays include a scintillation material attached to the photodiode array. The scintillation material converts high energy photons (x-rays and gamma rays) into visible or near visible light. This light is then detected by photodiodes in the photodiode array. Scintillation light impinging on each photodiode of the photodiode array is converted thereby into an electrical signal which is amplified and measured to provide an indirect measurement of the incident photon flux.
The operation of a scintillator photodiode array requires a constant influx of photons to deliver a constant scintillation light output (from many photons), thereby producing a photodiode output current that is relatively proportional to the incoming photon flux rate. Scintillator photodiode arrays, therefore, indirectly measure the photon flux by detecting the light emitted by the scintillation material. Consequently, scintillator photodiode arrays do not have the capability to count photons or provide energy information about the detected photons. Notwithstanding, due to their indirect method of measuring photon flux, photodiode arrays can produce a nearly linear response to increasing photon flux.
In contrast to a scintillator photodiode array detector, an energy discriminating photon counting detector can be used to count and discriminate each incoming photon and its energy. A typical photon counting detector includes an array of semiconductor detector elements, e.g., without limitation, CdZnTe, (or pixels) and signal processing electronics. When radiation, such as, without limitation, an x-ray or gamma ray, strikes one of the pixels, charge is generated that is proportional to the energy of the radiation event. The charge generated by the pixel is output thereby as a current or voltage pulse. The radiation event is characterized by the location of the detector element in the array thereof which is struck and the energy of the radiation event. A controller determines this information for every radiation event for all the pixels, accumulates the radiation events occurring during a sample interval of time for all of the pixels into a window or frame, temporarily stores the window or frame in digital form and processes the digital window or frame to form an image.
More specifically, the current or voltage pulse output by each pixel in response to a radiation event (an incoming photon) is compared by a comparator (either directly or after amplification) to a threshold voltage or current. Current or voltage pulses below this threshold value are ignored. In contrast, a count of each current or voltage pulse exceeding this threshold value is accumulated by a controller for processing in a manner known in the art. The counts accumulated from all of the pixels of the photon counting detector for a specific sample interval of time can be converted by the controller in a manner known in the art into an electronic version of an image which can be displayed as a visual image on a display of a imaging system.
As can be seen, a photon counting detector ideally counts each photon individually. Thus, ideally, each voltage or current pulse and the corresponding count accumulated by the controller is in response to a single photon event and the rate that such current or voltage pulses are produced happens at time intervals that permit the comparator utilized to compare the threshold value to the current or voltage pulse to return to baseline between photon events. However, in practice, photons generated by x-ray and gamma ray sources are random in nature. Therefore, as the photon flux increases, the probability of two photons striking the same pixel at or near the same time also increases. In nuclear spectroscopy, this is commonly referred to as a “pulse pileup”.
With reference to FIG. 1, a graph 2 showing the output of a single pixel in response to photons striking a pixel of a photon counting detector is shown in relation to a graph 4 of an output of a comparator that is utilized to compare the output of the pixel (either directly or after amplification) to a threshold value 6. As can be seen in corresponding areas 8a and 8b of graphs 2 and 4, respectively, when two or more photons strike the pixel in a short interval of time, the signal output by the comparator does not return to a baseline value before another photon strikes the pixel, whereupon the amplitude of a pulse output by the pixel in response to the other photon striking the pixel is artificially increased by the residual amplitude of the pulse output by the pixel in response to the previous photon striking the pixel. While area 8b of FIG. 1 shows that pulses were counted, one or more of said pulses may not have been greater than or equal to threshold value 6 and, therefore, may not have been counted correctly.
Areas 9a and 9b show that two or more photon pulses received at substantially the same time will result in the output of the amplifier being above threshold 6 for the duration of both photons, whereupon there is no discrimination between each photon striking the corresponding pixel. Similarly, for areas 10a and 10b in FIG. 1.
Both photodiode arrays and photon counting detectors have pixel-to-pixel non-uniformities that produce slightly different sensitivities. The result of non-uniform pixel response to photon flux is lines or streaks in the resulting images. In a photodiode array, the linear response of the detectors allows for a single multiplication factor to be calculated for each pixel to bring all of the pixels in the array to the same mean value for a given constant flux rate. However, this is not the case with a photon counting system as the response is nonlinear and much more difficult to correct.